The application of electronic paper is proposed to various portable devices, such as electronic books, sub-displays for mobile terminal devices and display devices for IC cards. As one of major display devices of electronic paper, there is a display device using a liquid crystal composition in which a cholesteric phase is formed (referred to as cholesteric liquid crystal or chiral nematic liquid crystal, and in the present specification, the term of cholesteric liquid crystal is used integrally). The cholesteric liquid crystal has excellent features such as semi-permanent retention characteristic of display (memorability), characteristic of vivid color display, high contrast characteristic and high resolution characteristic.
FIG. 1 is a diagram illustrating a cross-sectional configuration of the liquid crystal display device using the cholesteric liquid crystal, by which a full color display can be made. A liquid crystal display device 1 has a laminated structure of blue color display section 10, green color display section 11 and red color display section 12, in order from the display surface on the user 3 side. In the above figure, the upper substrate side is the display surface, and it is configured that an external light 2 is incident from the upper side of the substrate toward the display surface.
The blue color display section 10 includes a liquid crystal 10LC for blue color, which is sealed in between a pair of upper and lower substrates 10A, 10B, and a drive circuit 10P for applying a predetermined pulse voltage to the liquid crystal layer 10LC for blue color. The green color display section 11 includes a liquid crystal 11LC for green color, which is sealed in between a pair of upper and lower substrates 11A, 11B, and a drive circuit 11P for applying a predetermined pulse voltage to the liquid crystal layer 11LC for green color. Further, the red color display section 12 includes a liquid crystal 12LC for red color, which is sealed in between a pair of upper and lower substrates 12A, 12B, and a drive circuit 12P for applying a predetermined pulse voltage to the liquid crystal layer 12LC for red color. Moreover, on the back face of the lower substrate 12B of the red color display section 12, a light absorption layer 13 is disposed.
The cholesteric liquid crystal used in each of the liquid crystal layers 10LC, 11LC and 12LC for blue, green and red colors is the mixture of a liquid crystal having a relatively large amount of chiralic additive agent (which is also referred to as chiral material) being added to a nematic liquid crystal, with a content rate of several tens wt %. When the relatively large amount of chiral material is contained in the nematic liquid crystal, it is possible to form a cholesteric phase in which nematic liquid crystal molecules are intensively twisted into a spiral shape. For the above reason, the cholesteric liquid crystal is also referred to as chiral nematic liquid crystal.
The cholesteric liquid crystal provides a bistable property (memory characteristic), and can take either one of the states of a planar state (reflection state), a focal conic state (transmission state) and an intermediate state by the mixture thereof, by the control of an electric field intensity applied to the liquid crystal. Further, once the cholesteric liquid crystal takes the planar state, the focal conic state or the intermediate state thereof, the above state is stably retained even the electronic field is removed thereafter.
For example, the planar state is obtained by applying a strong electronic field to a liquid crystal layer by the application of a predetermined high voltage between the upper and the lower substrates, so that the liquid crystal is made to be a homeotropic state, and thereafter, the electric field is abruptly removed to zero. Also, the focal conic state is obtained, for example, by applying an electric field to the liquid crystal layer by the application of a predetermined voltage, which is lower than the above high voltage, between the upper and the lower substrates, and thereafter, the electric field is abruptly removed to zero. Or otherwise, the focal conic state may also be obtained by gradually applying a voltage from the planar state. Further, the intermediate state between the planar state and the focal conic state is obtained, for example, by applying an electronic field to the liquid crystal layer by applying a voltage, which is lower than the voltage to obtain the focal conic state, between the upper and the lower substrates, and thereafter, the electric field is abruptly removed to zero.
FIGS. 2A and 2B are diagrams illustrating the display principle of the liquid crystal display device by use of the cholesteric liquid crystal. In FIG. 2, the blue color display section is explained as an example. FIG. 2A depicts the orientation states of the liquid crystal molecules LC of the cholesteric liquid crystal, when the liquid crystal layer 10LC for blue color in the blue color display section 10 is in the planer state. As depicted in FIG. 2A, the liquid crystal molecules LC in the planar state form a spiral structure by the successive rotation thereof in the substrate thickness direction. The spiral axis of the spiral structure is substantially perpendicular to the substrate plane.
In the planar state, a light having a predetermined wavelength corresponding to the spiral pitch of the liquid crystal molecules is selectively reflected on the liquid crystal layer. Let n to be an average refractive index of the liquid crystal layer, and also let p to be a spiral pitch, then a wavelength λ producing a maximum reflection is expressed by λ=n·p. Accordingly, if the average refractive index n and the spiral pitch p are determined so as to obtain λ=480 nm, the liquid crystal layer 10LC for blue color in the blue color display section 10 selectively reflects a blue light when being in the planar state. The average refractive index n can be adjusted by the selection of the liquid crystal material and the chiral material, and the spiral pitch p can be controlled by the adjustment of the content rate of the chiral material.
FIG. 2B depicts the states of orientation of liquid crystal molecules in the cholesteric liquid crystal, when the liquid crystal layer LC for blue color in the blue color display section 10 is in the focal conic state. As depicted in FIG. 2B, the liquid crystal molecules in the focal conic state form a spiral structure by the successive rotation thereof in the direction of the substrate plane, and thus, the spiral axis of the spiral structure becomes substantially parallel to the substrate plane. In the focal conic state, the selectivity of the reflective wavelengths is lost in the liquid crystal layer 10LC for blue color, and most of the incident light 2 is transmitted through. Then, the transmitted light is absorbed in the light absorption layer 13 being disposed on the back plane of the lower substrate 12B in the red color display section 12, and accordingly, a dark color (black) display is produced.
In the intermediate state between the planar state and the focal conic state, it is possible to vary the intensity of the reflected light because a ratio between the reflected light and the transmitted light can be adjusted according to the state thereof. As such, in the cholesteric liquid crystal, the amount of the reflected light can be controlled by the state of orientation of the liquid crystal molecules being twisted in a spiral shape.
If a cholesteric liquid crystal, which selectively reflects green or red light in the planar state, is sealed into each of the liquid crystal layer for green color and the liquid crystal layer for red color, like the liquid crystal layer for blue color, a liquid crystal display device of full color display can be realized.
Thus, by using the cholesteric liquid crystals, and by laminating the liquid crystal display panels each selectively reflecting red, green or blue light, a full-color display device having memory characteristics can be obtained. The above color display can be made with power consumption=0, except for the time of rewriting the screen.
FIG. 3 is a diagram illustrating a reflectivity characteristic versus a drive voltage in a cholesteric liquid crystal. When a strong electric field (by a high voltage V1) is given to the liquid crystal, there is produced a homeotropic state HT, in which the spiral structure of the liquid crystal molecules is entirely released, and the entire molecules are subject to the direction of the electric field. If the electric field is abruptly removed to zero from the homeotropic state HT, the spiral axis of the liquid crystal becomes perpendicular, and a planar state PL is produced accordingly. Then, if the electric field is removed after a weak electric field (by a voltage V2) in the order insufficient to release the spiral structure of the liquid crystal molecules is applied from the planner state PL, a focal conic state FC is produced. Further, if an abrupt electric field removal is made after the supply of intermediate electric fields (by voltages V3, V4), there is produced a gray state, in which the planar state and the focal conic state are existent in a mixed manner.
In case that the liquid crystal is driven by a pulse voltage, when the initial state is the planar state PL, the focal conic state FC can be produced by letting the pulse voltage to be the voltage V2, or of that order, and the planar state PL can be produced by further letting the pulse voltage to be the higher voltage V1. Also, when the initial state is the focal conic state FC, the focal conic state FC can be maintained by letting the pulse voltage to be the voltage V2, or of that order, and the planar state PL can be produced by further letting the pulse voltage to be the higher voltage V1. Further, by applying the voltages V4, V3 of gray ranges A, B from the planar state PL, it is possible to produce gray states in which the planer state and the focal conic state are existent in a mixed manner.
In the liquid crystal display device depicted in FIG. 1, by writing an image having the planar state (reflection state, RGB) and the focal conic state (transmission state, black) into each RGB display panel 10, 11, 12, it is possible to obtain a multi-color display having eight colors. In the above case, it is sufficient to write a monochrome image with the voltage V1 or V2 depicted in FIG. 3 applied to each display panel, and accordingly, very small energy is required for writing, and a required accuracy for the power voltage is low.
On the other hand, when performing full color display exceeding eight colors, it is necessary to write a multi-grayscale image into each display panel. In the patent document WO 06/103738 (Oct. 5, 2006), there is described a drive method for writing a multi-grayscale image in a cholesteric liquid crystal display panel.
According to the above display method for full color display, in a step 1, either a high voltage or a low voltage is applied to the liquid crystal of each pixel, so as to produce a planar state or a focal conic state. Further, in a step 2, a relatively high or low voltage is applied to a pixel in the planar state, so that a gray state is produced. By the execution of the drive process in the step 2 for a plurality of times, it is possible to produce gray states having multi-grayscale levels. According to the above write drive method, the gray states are realized with high accuracy, and it is possible to provide electronic paper for displaying a color image of high quality.